1. Field of the Invention
The present invention relates generally to millimeter-wave phase shifters, and more particularly to monolithic millimeter-wave phase shifters using optically activated superconducting switches.
2. Description of the Related Art
Phased-array antennas are increasingly being used in radar systems, e.g. mm-wave radar systems, and in communications systems, e.g. satellite communication systems. These antennas permit electronic beam steering, obviating the need for mechanical pointing. In order to utilize a phased-array antenna, these system must include a phase shifter, which provides incremental offset phase excitation to each element in the array.
Though many types of phase shifters exist, the most desirable is the true time delay type, which can provide frequency independent steering for large arrays. The most popular true time delay phase shifter utilizes a switched line technique. This technique achieves true time delay by switching an input signal between two alternative routes, i.e., a reference path and a delay path. The delay path has a transmission length that is longer than that of the reference path by a portion of the wavelength of the input signal, and therefore causes the signal transmitted therethrough to exit out-of-phase relative to the signal transmitted through the reference path. Field effect transistor (FET) switches or diode switches have been utilized to switch the signal between the reference path and the delay path. Four of these switches are required per bit.
These conventional phase shifters include the following disadvantages: asymmetric insertion loss between the delay path and the reference path, high overall insertion loss, poor isolation between the radio frequency signal and switch bias signals, and signal distribution complexity. The difference in insertion loss between the delay path and the reference path results primarily from variation in the shunt capacitance of the switches during their on and off states. Some investigators have attempted to resonate out the shunt capacitance of the switches during their off state using parallel inductors. This approach has met with only limited success and is difficult to achieve at millimeter wavelengths. The lack of insertion loss symmetry degrades system performance and requires complex and expensive compensation devices. Generally, asymmetric insertion loss due to the different transmission lengths of the reference path and the delay path is minimal in comparison to that caused by variation in the shunt capacitance of the switches.
The total insertion loss of conventional phase shifters is large and is roughly proportional to the reactance associated with the switches, e.g., typically greater than 2 dB per bit at 30 GHz. Also, interaction between the radio frequency signal and the switch bias signals required to control the switches presents another problem. Standing-wave patterns are set-up on the biasing lines, resulting in unpredictable performance and sensitivity to bondwire lengths.
Also, because some of the systems in which conventional phase shifters are used may include hundreds of elements per array and perhaps five bits per element, and because each bit requires a minimum of two control lines to toggle the switches, signal distribution is complex.